In many types of testing operations and particularly in non-destructive testing, test results are indicated by the pattern of movement of the flying spot of a cathode ray tube. For example, in eddy current testing systems of types known in the art, an eddy current probe is moved over the surface of a part while DC output signals are developed in proportion to quadrature phase components of a signal derived from the probe. Such DC signals are applied to the horizontal and vertical inputs of a cathode ray tube display section to cause a spot to be in a corresponding position. From the pattern of movement of the spot as the probe moves over the part, the characteristics of the part and the nature of defects therein can be accurately ascertained. It is frequently desirable to be able to store the pattern for more careful analysis thereof, photographing and the like. Heretofore, cathode ray tubes have been used in which long persistance phosphors are incorporated in the screens thereof to store patterns for long periods of time. Such tubes have been generally satisfactory but have had disadvantages, particularly in that they are quite expensive and have a limited life. Frequently, the storage of patterns is uneven and lines produced are ragged with fuzzy edges. Also, the time in which signals may be stored is not unlimited.
Oscilloscopes are available in which signals may be stored digitally, but such are not suitable for testing systems in which a pattern is to be produced from a pair of analog signals, the oscilloscopes being operable with a time base versus vertical amplifier signal format.